This invention relates to thermotropic liquid crystalline polymer compositions having a high tracking index and excellent flame-retarding properties.
Liquid crystalline polymers (LCPs) have become important items of commerce, being useful as molding resins for general purpose uses, and more specifically in the electrical and electronics industries due to their thermal stability, chemical resistance, and other desirable properties. For many electrical and electronics applications, the molding resins should exhibit good tracking resistance and good flame-retarding properties.
Tracking is a phenomenon associated with the formation of permanent and progressive conducting paths on the surface of materials by the combined effects of an electrical field and external surface pollution. Electrical tracking can occur when a damaged energized electrical part becomes wet, e.g., from electrolytes or condensation. This tracking may lead to flash over and arcing that causes further damage in the electrical part, causing a catastrophic cascade failure. Tracking can occur at low voltages, e.g., 100V AC or less but becomes less likely as the voltage is reduced. The comparative tracking index (CTI) rating provides a quantitative indication of a composition""s ability to perform as an electrical insulating material under wet and/or contaminated conditions. In determining the CTI rating of a resin composition, two electrodes are placed on a molded test specimen. A voltage differential is then established between the electrodes while an aqueous ammonium chloride solution is slowly dripped on the test specimen. The CTI rating of the test specimen is the maximum electrode voltage differential at which, in five consecutive tests, more than 50 drops of the solution must be applied to the test specimen in order to cause tracking to occur. Hence, the CTI value is the highest voltage at which a molding is found not to exhibit conductivity. For some applications in the electrical and electronics industry, the CTI value is expected to be at least 220 volts.
Flame-retarding properties are measured according to the Underwriters"" Laboratories Bulletin No. 94 test, in which a sample having approximate dimensions of 2.5xe2x80x2 by 0.5xe2x80x2 by 0.0625xe2x80x2 is contacted with a Bunsen burner flame for 10 seconds. The sample is then removed and the amount of time it takes for the flame to be out is recorded. The test also characterizes the material as xe2x80x9cdrippingxe2x80x9d or xe2x80x9cnon-dripping,xe2x80x9d since flaming drops of resin which could cause adjacent structures to bum are of concern. A sample is said to xe2x80x9cdripxe2x80x9d when some molten resin falls off the bar in the test. If the drip is burning, and it burns enough to ignite a cotton ball located beneath the bar, the material fails the test. Even self-extinguishing drips, which technically do not cause the material to fail the test, are considered undesirable in practice. The test is repeated with the same sample placed in contact with a Bunsen burner flame again for 10 seconds. A UL V-0 rating requires a flame-out-time (FOT) of not more than 10 seconds for any one sample in a test of five samplesxe2x80x94each repeated twice, and a cumulative FOT of not more than 50 seconds total for the five samples. When the five test samples have a cumulative FOT of 50 seconds and one or more of the five samples fails the 10-second test, than a V-1 rating is granted. A V-1 rating is universally required for electrical applications with V-0 being necessary, although flame retardants having V-1 or V-2 ratings are also suitable for many end uses. However, as a practical matter, a V-0 rating with a maximum cumulative FOT of 35 seconds is often mandated by purchasers.
It is known that halogen-containing compounds, such as decabromodiphenylethane or brominated polystyrene, can be used to prepare flame-retardant thermoplastics. U.S. Pat. No. 4,824,723 discloses a flame resistant electrically insulating multilayer material comprising a core of a thermoplastic polymer and blends and an outer layer comprising blends of the polymers with a halogen-containing flame-retardant, with a V-0 rating and an excellent CTI rating of greater than 500 volts.
U.S. Pat. No. 4,636,544 discloses the use of titanium dioxide (TiO2) to improve the CTI of resin molding compositions. However, a flame retardant is still required in the composition. TiO2 is used in a limited amount, preferably in an amount of about 1 to about 10 parts TiO2 for each 85 parts by weight of resin and halogenated poly(1,4-butylene terephthalate) composition. U.S. Pat. No. 4,421,888 discloses an article molded from a non-burning, non-dripping filled polyester composition having a high level of tracking resistance, comprising a mixture of polyethylene terephthalate and polybutylene terephthalate, a mixture of fiber glass and talc, and an effective amount of a halogen-containing flame-retardant.
The disadvantage of flame-resistant agents, particularly flame-resistant agents based on halogenated hydrocarbons in thermoplastics and particularly in polyesters, is that they cause the electrical properties to deteriorate. Additionally, the use of halogen-containing compounds in thermoplastics is severely restricted in certain countries for the reason that they give off toxic fumes while burning, and the halogenated flame retardants themselves may be toxic. A need presently exists for plastic compositions which possess the thermal stability, chemical resistance, and other desirable characteristics of LCPs as well as excellent flame-resistant property and high CTI rating without the use of halogen-containing compounds as flame-retardants.
Surprisingly, we have found that good tracking index and flame resistance can be achieved with other desirable characteristics of LCPs in a resin composition without the use of any flame-retardants. The novel composition consists essentially of:
a) a wholly aromatic polyester which is melt processible and which displays anisotropy in the molten state;
b) a non-conductive filler material having a mean particle size of less than about 4 xcexcm, present in an amount sufficient to increase the comparative tracking index (CTI) rating of said composition to above 220 volts and render the composition non-burning; and optionally
c) at least one fluorescent optical brightener in an amount of greater than 0.005 wt. % and having a boiling point of Tbp greater than Tmxe2x88x9260xc2x0 C., where Tm is the melting point of the LCP.
The extraordinarily small amount of fluorescent brightener in combination with the non-conductive filler to the LCP resin composition was found to surprisingly and significantly improve the flame-retardant property of the resin. Compositions with and without at least one fluorescent optical brightener are included within the invention.
The invention also relates to a practical method for the production of a resin composition for electrical and electronics application having good tracking index as well as flame-resistant properties by blending a wholly aromatic polyester with a non-conductive filler material having a mean particle size of less than about 4 xcexcm, preferably a chloride-process rutile-type metal-oxides coated titanium dioxide and preferably in an amount of about 30 to 50 weight percent filler material based on total weight of said aromatic polyester and said filler material.
The present invention further relates to electrical devices, by fabricating the electrical devices from a resin composition consisting essentially of: a) a wholly aromatic polyester which is melt processible and which displays anisotropy in the molten state; b) a non-conductive filler material having a mean particle size of less than about 4 xcexcm; and optionally c) at least one fluorescent optical brightener in an amount of greater than 0.005 wt. % and having a boiling point of Tbp greater than Tmxe2x88x9260C, where Tm is the melting point of the LCP.
Also disclosed herein is a electronic or electrical apparatus, comprising a component formed from a liquid crystalline polymer composition consisting essentially of a) a thermotropic liquid crystalline polymer component which is an aromatic polyester, poly(ester-amide), poly(ester-imide), poly(ester-amide-imide), or mixtures thereof; b) at least one non-conductive filling agent component having a longest dimension of less than about 4 xcexcm; and optionally c) at least one fluorescent optical brightener in an amount of greater than 0.005 wt. % and having a boiling point of Tbp greater than Tmxe2x88x9260C, where Tm is the melting point of the LCP; and wherein said electronic or electrical apparatus has a comparative tracking index (CTI) rating above 220 volts and a flammability rating of V-0 in test UL-94 at a 0.0625xe2x80x3 thickness, and said apparatus also comprises an electrical conductor carrying a voltage of 200 volts or more.